Question

Considering the strength of the ligand, the highest excitation energy will be observed in:

• A. \([ \text{Co(H}_2\text{O)}_6 ]^{3+}\)
• B. \([ \text{Co(NH}_3)_6 ]^{3+}\)
• C. \([ \text{Co(CN)}_6 ]^{3-}\)
• D. \([ \text{CoCl}_6 ]^{3-}\)

Hint:

Stronger field ligands produce greater splitting (\(\Delta\)), increasing excitation energy. Use spectrochemical series for comparison.

Answer 1

The Correct Option is C

Ligand field strength increases in the order: \[ \text{Cl}^-<\text{H}_2\text{O}<\text{NH}_3<\text{CN}^- \] Excitation energy is directly related to the crystal field splitting energy \(\Delta\), which is maximum for strong field ligands like CN\(^{-}\). Therefore, complex with CN\(^{-}\) ligand will exhibit: - Maximum crystal field splitting - Highest excitation energy for d–d transition \[ \boxed{[ \text{Co(CN)}_6 ]^{3-}\ \text{has the highest excitation energy}} \]